1. Field of the Invention
The present invention relates to linear integrated circuit designs. In particular, the present invention relates to audio amplifiers having no turn-on pop noise.
2. Discussion of the Related Art
During the power-up phase of an audio amplifier circuit's operation, an abrupt change in the amplifier's output voltage can result in an undesirable "pop" noise. The occurrence of this pop noise is illustrated with reference to FIG. 1, which shows a conventional audio amplifier circuit 100. As shown in FIG. 1, output amplifier circuit 100 includes speaker 101, audio amplifier integrated circuit 140, capacitors 102, 110, 111 and 112, and resistors 103 and 104. An input audio signal is provided at terminal 114, and an output signal of audio amplifier integrated circuit 140 is provided at terminal 117 to drive speaker 101. In FIG. 1, the operation of audio amplifier integrated circuit 140 can be represented by operational amplifier 105, and on-chip resistors 106, 107 and 108.
In audio amplifier circuit 100, the non-inverting input terminal 116 of operational amplifier 105 is biased by a voltage having a value approximately one-half the supply voltage V.sub.cc. The voltage at non-inverting terminal 116 is provided by the voltage divider formed by resistors 106 and 108. When amplifier circuit 100 is powered-up, input terminal 116 is slowly charged up to the one-half V.sub.cc value by the RC filter formed by resistor 107 and capacitor 112. Ideally, the voltage at output terminal 117 of operational amplifier 105 rises smoothly at approximately the same rate as the bias voltage's rise at terminal 116. However, pop noise arises in audio amplifier circuit 100 because of the output circuit of operational amplifier 105.
FIG. 2 shows gain/output stage 200 of operational amplifier 105 of FIG. 1. As shown in FIG. 2, gain/output state 200 of operational amplifier 105 includes input or gain stage 200a and output stage 200b. Input stage 200a is a differential amplifier including input NPN transistors 203 and 204, current source 208, bias circuit 207, and output transistor 206. In input stage 200a, a differential voltage is received across terminals 201 and 202, and an amplified non-inverting output signal is provided at terminal 205. The signal at terminal 205 can be directly provided to output stage 200b, to control NPN transistor 218 which, in turn, controls PNP transistor 214 and NPN transistors 215 and 216. When a positive differential voltage is imposed across input terminals 201 and 202, NPN transistor 218 is turned on, pulling the base of PNP transistor 214 towards ground (i.e. supply voltage V.sub.EE), thereby providing base currents to NPN transistors 215 and 216 to pull output node 207 towards supply voltage V.sub.cc. The current in NPN transistor 218 is provided by current source 211 through NPN transistor Resistors 209 and 210, which is maintained in the "on" state and current limited by the voltage divider of resistors 209 and 210. Conversely, when a negative differential signal is imposed across input terminals 201 and 202, the output voltage at terminal 205 reduces the current in NPN transistor 218, which in turn pulls output node 217 towards V.sub.cc, as a result of the current in NPN transistors 212 and 213.
Output stage 200b, however, causes a pop noise during the power-up phase. The pop noise is caused because, when NPN transistors 215 and 216 switches on, upon the power-up phase, the voltage at output terminal 217 jumps abruptly from 0 volts (V.sub.EE) to the voltage corresponding to the sum of (i) the collector-emitter voltage (V.sub.CE) of NPN transistor 215, (ii) the base-emitter voltage (V.sub.BE) of NPN transistor 215, and V.sub.BE of NPN transistor 216. This voltage sum is approximately 1.4 volts. Pop noises are observed to occur for an abrupt voltage jump of 100 millivolts. Further, because NPN transistors 215 and 216 switches off when the voltage at output node 217 falls below 1.4 volts, the voltage swing of output terminal 217 is limited the low voltage end at 1.4 volts.
Input stage 200a of operational amplifier 105 is further limited because the differential amplifier of input stage 200a cannot start up from a zero common mode input voltage across input terminals 201 and 202. At a common mode zero volts input, both NPN transistors 203 and 204 are in the off-state.
There is an additional source of turn-on pop noise in audio amplifier 100 of FIG. 1. If power to audio amplifier 100 is switched off momentarily, but power is returned before capacitors 102, 110, 111 and 112.sup.1 are fully discharged, the remaining charge in any of capacitor 102, 110, 111 and 112 may cause a surge in voltage at output terminal 117, leading to pop noise. FNT .sup.1 Capacitor 102 is a blocking capacitor for filtering out a DC component of the output signal at terminal 117. Capacitor 111 adjusts the gain of operational amplifier 105. Capacitor 112 prevents high frequency noise in the bias voltage at terminal 116. Capacitor 110 is a blocking capacitor for filtering out a DC component of the input signal at terminal 114.